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2.008-spring-2004 S.G. Kim 1

2.008 Design & Manufacturing II

Spring 2004

MEMS I

2.008-spring-2004 S.G. Kim 2

March 10thAsk “Dave” and “Pat”

Petty money up to $200, GogglesPlant tour, April 21, 22, sign up! By 4/2

Quiz 1 on March 17th

HW#4 due by Monday’s lecture75 minutes (45 min)

MEMS 1 today

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Elephant vs. Ant

Shock and impactScale and form factorLoad carrying capability

Spider silk v.s. steel

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Frog, Water Strider, Gecko

2.008-spring-2004 S.G. Kimhttp://robotics.eecs.berkeley.edu/~ronf

/GECKO/Figures/Hierarchy3.jpg5

Never try to mimic the nature.

e.g. Biomimetic researches.DNA

~2-1/2 nm diameter

Things Natural Things Manmade

MicroElectroMechanical devices10 -100 µm wide

Red blood cellsPollen grain

Fly ash~ 10-20 µm

Atoms of siliconspacing ~tenths of nm

Head of a pin1-2 mm

Quantum corral of 48 iron atoms on copper surfacepositioned one at a time with an STM tip

Corral diameter 14 nm

Human hair~ 10-50 µm wide

Red blood cellswith white cell

~ 2-5 µm

Ant~ 5 mm

The Scale of Things -- Nanometers and More

Dust mite

200 µm

ATP synthase

~10 nm diameter Nanotube electrode

Carbon nanotube~2 nm diameter

Nanotube transistor

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O OO O OO OO

O

S

O

S

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21st Century Challenge

Combine nanoscale building blocks to make novel functional devices, e.g., a photosynthetic reaction center with integral semiconductor storage

The

Mic

row

orld

0.1 nm

1 nanometer (nm)

0.01 µm10 nm

0.1 µm100 nm

1 micrometer (µm)

0.01 mm10 µm

0.1 mm100 µm

1 millimeter (mm)

1 cm10 mm

10-2 m

10-3 m

10-4 m

10-5 m

10-6 m

10-7 m

10-8 m

10-9 m

10-10 m

Visib

le

The

Nan

owor

ld

1,000 nanometers =

Infra

red

Ultra

violet

Micr

owav

eSo

ft x-

ray

1,000,000 nanometers =

Zone plate x-ray “lens”Outermost ring spacing

~35 nm

Office of Basic Energy SciencesOffice of Science, U.S. DOE

Version 03-05-02

DOE 2001

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Transition: Micro to Nano20th Century - Microelectronics and Information Technology

Semiconductors, computers, and telecommunication

21st Century - Limits of Microsystems Technology--- Nanotechnology

Moore’s lawHard disc drive

John Bardeen, Walter Brattain, and William Shockleyat Bell Laboratories, “First Transistor” 2.008-spring-2004 S.G. Kim 8

Moore’s Law

Year of introduction Transistors

4004 1971 2,250

8008 1972 2,500

8080 1974 5,000

8086 1978 29,000

286 1982 120,000

386™ 1985 275,000

486™ DX 1989 1,180,000

Pentium® 1993 3,100,000

Pentium II 1997 7,500,000

Pentium III 1999 24,000,000

Pentium 4 2000 42,000,000

The number of transistors per chip doubles every 18 months. – Moore’s Law

_ Rock’s Law

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Microelectronics Technology

To meet the Moore’s Law,

line width(1/2 pitch) requirement

100 nm 2005

70 nm 2008

50 nm 2011

35 nm 2014

The International Technology Roadmap for Semiconductors, 1999

No solution yet, nanolithography?

G-line:436nmI-line: 365nmDUV: 248 nm

193 nmEUV

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Aerial density, hard disk

Superparamagnetic Effect“a point where the data bearing particles are so small that random atomic level vibrations present in all materials at room temperature can cause the bits to spontaneously flip their magnetic orientation, effectively erasing the recorded data. “

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A DNA molecule is 2.5 nm wide.

What is Nanotechnology?

Nanomanufacturing?

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Nano in MEFluidics, heat transfer and energy conversion at the micro- and nanoscaleBio-micro-electromechanical systems (bio-MEMS)Optical-micro-electromechanical systems (optical-MEMS)Engineered nanomaterialsNano manufacturing

Course 2A (Nanotrack)

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MEMSMEMS MEMSTechnologiesOptical MEMSRF MEMSRF MEMSData StorageBio. MEMSPower MEMSMEMS for ConsumerElectronicsMEMS In Space

MEMS for Nano.

Courtesy: Sandia national laboratory

MaterialsProcessesSystems

http://www.memsnet.org/mems/what-is.html

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MEMS (Microelectromechanical Systems)

Intergrated systems of sensing, actuation, communication, control,power, and computingTiny,Cheaper,Less powerNew functions!!! (chemical, bio, µ-fluidic, optical, …)

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Tiny ProductsDLP (Digital Micromirror Array)

DLP106 micromirrors, each 16µm2, ±10° tilt

(Hornbeck, Texas Instruments DMD, 1990)

TM

2.008-spring-2004 S.G. Kim Analog Devices16

Tiny ProductsAirbag sensors: Mechanical vs. MEMS

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Tiny ProductsAirbag sensors: Mechanical vs. MEMSDLP (Digital Micromirror Array)DNA chipOptical MEMS

Smalltimes, Vol.2 , no. 6, 2002

Tiny Tech venture funding, 2002

2.008-spring-2004 S.G. Kim D. Kamen18

Segway

-Tilt-Rotation

(Courtesy of Segway (r) Human Transporter (HT). Used with permission.)

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Vibrating Gyroscope

By Charles Stark Draper LaboratoryBy Charles Stark Draper Laboratory

x

z

y

CoriolisAcceleration

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Electrostatic Comb Drive/sensing

Paralle Plate CapacitorCapacitance=Q/V=ε A/dε Dielectric permittivity of air

Electrostatic Force = ½ ε (A/d2).V2

Pull-in point: 2/3 d

d

K

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Comb DriveC= ε A/d = 2n ε l h/d∆C = 2n ε ∆l h/d

Electrostatic force

Fel = ½ dC/dx V2 = n ε h/d V2

∆l

d

V

x

l

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Suspension mode failures

xy

x

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Comb Drive Designs

DC Bias

AC Signal(180° phase shift)

AC Signal

Grating beamsFlexuresElectrostatic comb-drives

linear rotational

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Capacitive Accelerometercapacitivesensor

platemass meander

spring

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Microfabrication process flow

Single-mask process

IC compatible

Negligible residual stress

Thermal budget

Not yet packaged

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1) Begin with a bonded SOI wafer. Grow and etch a thin thermal oxide layer to act as a mask for the silicon etch.

2) Etch the silicon device layer to exposethe buried oxide layer.

3) Etch the buried oxide layer in buffered HF to release free-standing structures.

Si device layer, 20 µm thickburied oxide layer

Si handle wafer

oxide mask layer

silicon

Thermal oxide

SOI (Silicon on insulator)

2.008-spring-2004 S.G. Kim G. Barbastathis & S. Kim27

Surface micromachinedStructure 2 µm

Vertical stiction

DRIE micromachinedStructure 10 µm

Lateral stiction

300 µm

No stiction

DRIE micromachinedStructure 10 µm

Problems of fabrication

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ADXL 50 accelerometer

Capacitive sensing

Comb drive

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Process Flow

Deposition Lithography Etch

Wafers DevicesPhoto resist coatingPattern transferPhoto resist removal

OxidationSputteringEvaporationCVDSol-gelEpitaxy

Wet isotropicWet anisotropicPlasmaRIEDRIE

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Micromachining processes•Bulk micromachining•Surface micromachining•Bonding•LIGA

• x-ray lithography, electrodeposition and molding

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LIGA process

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Bulk, Surface, DRIEBulk micromachining involves removal of the silicon wafer itself

Typically wet etchedInexpensive equipmentsIC compatibility is not good.

Surface micromachining leaves the wafer untouched, but adds/removes additional thin film layers above the wafer surface.

Typically dry etchedExpensive equipmentsIC compatibility, conditionally.

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MaterialsMetals

Al, Au, ITO, W, Ni, Ti, TiNi,…Insulators

SiO2 - thermally grown above 800oC or vapor deposited (CVD), sputtered. Large intrinsic stress

SixNy – insulator, barrier for ion diffusion, high E, stress controllable

Polymers: PR, SU-8, PDMS

Glass, quartzSilicon

stronger than steel, lighter than aluminumsingle crystal, polycrystalline, or amorphous

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Silicon

Atomic mass average: 28.0855Boiling point: 2628KCoefficient of linear thermal expansion: 4.2. 10-6/°CDensity: 2.33g/ccYoung’s modulus: 47 GPaHardness scale: Mohs’ 6.5Melting point: 1683K Specific heat: 0.71 J/gK

Electronic grade silicon99.99999999% purity

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MaterialsSingle crystal silicon

Anisotropic crystalSemiconductor, great heat conductor

Polycrystalline silicon – polysiliconMostly isotropic materialSemiconductor

SemiconductorElectrical conductivity varies over ~8 orders of magnitude depending on impurity concentration (from ppb to ~1%)N-type and P-type dopants both give linear conduction.

Two different types of dopingElectrons (negative, N-type) --phosphorusHoles (positive, P-type) --boron

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Silicon Ingot

Czochralski (CZ) method

Float Zone (FZ) method.

1” to 12” diameterhttp://www.msil.ab.psiweb.com/english/msilhist4-e.html

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Miller indices, plane

[100]

[010]

[001]

a

b

c

[abc]

1/a1/b

1/c (abc)

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Crystallographic planes

[100]

[010]

[001]

(100)

{100}(001)

(010)

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Miller indices

[100]

[010]

[001]

ab

c

[abc]

•[abc] in a cubic crystal is just a directional vector•(abc) is any plane perpendicular to the [abc] vector•(…)/[…] indicate a specific plane/direction•{…}/<…> indicate equivalent set of planes/directions

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Wafers of different cuts

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Crystallographic planes

[100]

[010]

[001]

(110)(111)

(001)

54.74o

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Isotropic silicon etchantsHNA (“poly-etch”) -wet

Mix of HF, nitric acid (HNO3), and acetic acids (CH3COOH)

Difficult to control etch depth and surface uiformityXeF2 -dry

gas phase, etches silicon, polysiliconDoes not attack SiO2, SiNx,metals, PR

Etching

Wet

Dry

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Anisotropic wet etching

Many liquid etchants demonstrate dramatic etch rate differences in different crystal directions

<111> etch rate is slowest, <100> fastestFastest: slowest can be more than 100:1KOH, EDP, TMAH most common anisotropic silicon etchantsPotasium Hydroxide (KOH), TetramethylAmmonium Hydroxide (TMAH), and Ethylene Diamine Pyrochatecol (EDP)

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KOH EtchingEtches PR and Aluminum instantly(100) to (111) 100 to 1 etch rate

V-grooves, trenchesConcave stop, convex undercutCMOS incompatible

Masks:SiO2: for short period

SixNy: Excellent

heavily doped P++ silicon: etch stop

<111><100>

Silicon Substrate

54.7

a 0.707a

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Anisotropic wet etching

When a (100) wafer with mask featuresOriented to <110> direction is placed in an anisotropic etchant.

A square <110> oriented mask feature results in a pyramidal pit.

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Anisotropic wet etching

When a (100) wafer with mask featuresOriented to <110> direction is placed in an anisotropic etchant.

A square <110> oriented mask feature results in a pyramidal pit.

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Dry etchingRIE (reactive ion etching)

Chemical & physical etching by RF excited reactive ionsBombardment of accelerated ions, anisotropicSF6 Si, CHF3 oxide and polymersAnisotropy, selectivity, etch rate, surface roughness by gas concentration, pressure, RF power, temperature control

Plasma etchingPurely chemical etching by reactive ions, isotropic

Vapor phase etchingUse of reactive gases, XeF2No drying needed

sticktion

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DRIE (Deep RIE)Alternating RIE and polymer deposition process for side wall protection and removalEtching phase: SF6 /ArPolymerization process: CHF3/Ar forms Teflon-like layerInvented by Bosch, process patent, 1994

-1.5 to 4 µm/min-selectivity to PR 100 to 1

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1 µm

Scalloping and Footing issues of DRIE

Scalloped sidewall

Top wafer surfacecathode Top wafer surface

anode

Tip precursors

Scalloped sidewall

Top wafer surfacecathode Top wafer surface

anode

Tip precursors

<100 nm silicon nanowire over >10 micron gap

Footing at the bottom of

device layerMilanovic et al, IEEE TED, Jan. 2001.

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Deep Reactive Ion EtchSTS, Alcatel, Trion, Oxford Instruments …

Most wanted by many MEMS students

High aspect ratio 1:30

Easily masked (PR, SiO2)